The invention relates to a television signal transmission system in which a television signal is transmitted as a sequence of television frames each frame of which comprises a plurality of television lines with substantially all the lines of a frame having first and second time multiplexed portions, the first portion of each of said lines conveying sound/data information in digital form at a given bit rate whilst the second portion of each line outside the vertical blanking interval or intervals conveys vision information. The invention also relates to apparatus for use with such a system.
Transmission systems of the above type are described in the European Broadcasting Union (EBU) document Tech. 3258-E "Specification of the systems of the MAC/packet family" which is dated October 1986 and is incorporated herein by way of reference. This document describes a number of transmission systems of the MAC (multiplexed analogue components) type where the first portion of the majority of the lines of a frame forms a data burst of a given duration containing digital data, which can include sound, organised in packets whilst the second portion, outside the vertical blanking intervals, contains the compressed luminance and chrominance components, time multiplexed, of the vision information. Of the systems described the C-MAC/packet system has a digital bit rate of 20.25 Mb/s with the data burst being 2-4 PSK modulated and capable of conveying the equivalent of eight high quality sound channels whilst the vision information is frequency modulated. The C-MAC/packet system is intended for use in a direct broadcasting satellite (DBS) channel. The described D-MAC/packet system is intended to convey the full information of the C-MAC/packet system in distribution systems such as cable and in this system the data burst uses duobinary coding whilst the system uses amplitude modulation. It has already been proposed elsewhere that the D-MAC/packet system could be used for transmission via a satellite with the vision information being frequency modulated. The other system described in the above document is the D2-MAC/packet system in which the data in the data burst is again duobinary coded but the bit rate is reduced to 10.125 Mb/s such that it is capable of conveying the equivalent of four high quality sound channels. If this system is to be used in a satellite channel then frequency modulation is employed whilst if the system is to be used in a channel of restricted bandwidth e.g. in a distribution system then amplitude modulation is employed.
There have been recent proposals to transmit high definition television signals using one of the MAC/packet systems these proposals being generally referred to as HD-MAC. At the same time certain broadcasting organisation have announced their intention to commence their DBS services using the reduced specification D2-MAC/packet system.